The present invention relates to a developer for presensitized plates (hereunder referred to as "dry PS plate") for use in making a lithographic printing plate which requires no dampening water during printing (hereunder referred to as "dry lithographic printing plate") wherein silicone rubber layer serves as an ink-repellent layer and more specifically to a developer for dry PS plates, which is excellent in safety, developing properties and dot reproduction.
There have been proposed a variety of dry PS plates having a silicone rubber layer serving as an ink repellent layer. Among these, those disclosed in Japanese Patent Publication for Opposition Purpose (hereinafter referred to as "J.P. KOKOKU") Nos. 54-26923; 55-22781 and 56-23150 comprise a substrate provided thereon with a photosetting light-sensitive layer and a silicone rubber layer in order and the lithographic printing plate obtained therefrom makes it possible to obtain several tens of thousands of printed matters without using dampening water.
The aforesaid dry PS plate is in general processed through exposing and developing processes detailed below to form a lithographic printing plate.
First, a dry PS plate is exposed to actinic rays through an original film which is closely placed in contact with the plate in vacuo. Then, the exposed PS plate is immersed in a paraffinic hydrocarbon or a developer mainly comprised of the same. As a result, the silicone rubber layer in non-exposed areas greatly gets swollen with the developer and thus only the silicone rubber layer in the non-exposed areas is removed with or without the dissolution of the light-sensitive layer of the non-exposed areas. These portions from which the silicone rubber layer is removed serve as image areas. On the other hand, the silicone rubber layer in the exposed areas slightly get swollen with the developer, but they are not removed even when strongly rubbed with a pad containing the developer and remain on the plate surface since it is strongly adhered to the light-sensitive layer. Such portions serve as ink-repellent non-image areas. Thus, a dry lithographic printing plate is produced.
In general, components for swelling the silicone rubber layer (hereunder referred to as "silicone rubber swelling agent") and components for dissolving or swelling the light-sensitive layer beneath the silicone rubber layer (hereunder referred to as "developing agent") are possibly used as essential components of a developer for developing such a dry PS plate provided with a silicone rubber layer as an ink-repellent layer. Examples of the silicone rubber swelling agents include a single solvent system such as paraffinic hydrocarbons e.g., pentane, hexane, heptane, octane, nonane and decane; a mixture of isomers thereof or the like; fractional distillates of petroleum corresponding to mixture thereof, for instance, Isopar G and Isopar H (available from ESSO Petroleum Co., Ltd.). In addition, examples of effective developing agents are alcohols, esters, ketones, ethers, aromatic hydrocarbons, halogenated hydrocarbons and carboxylic acids.
A conventional developer for dry PS plates provided with a silicone rubber layer as the upper layer comprises a silicone rubber swelling agent as a basic component and a developing agent. Therefore, it contains the silicone rubber swelling agent in an amount greater than that required for swelling the silicone rubber layer and for this reason, the silicone rubber layer in the non-image areas likely causes scratches during development. In addition, since this silicone rubber swelling agent is composed of a paraffinic hydrocarbon, there is a risk of causing fire for any possible reason. In particular, when an automatic developing machine is used, it is required to use an automatic developing machine provided with a means for preventing firing. However, the use of such an automatic developing machine is not economical since it is very expensive compared with that for presensitized plates (conventional PS plates) which can be processed with a usual water-based developer.
To solve the foregoing problems, Japanese Patent Unexamined Publication (hereinafter referred to as "J.P. KOKAI") No. 61-275759 (which corresponds to U.S. Ser. No. 807,469 filed on Dec. 10, 1985 and now abandoned and DE No. 3,543,961A) proposes a developer for dry PS plates which comprises an organic solvent capable of swelling a silicone rubber layer, a surfactant and not less than 30% by weight of water. This developer does not cause scratches on the silicone rubber layer in non-image areas during development because of the presence of a desired amount of water. Moreover, its ignition point is higher than those of conventional developers free from water or developers having a low water content and it also exhibits high safety. Therefore, the automatic developing machine is not provided with a means for preventing firing and is less expensive.
However, in such a developer for dry plates exhibiting satisfactory properties, the silicone rubber swelling agent, the developing agent for dissolving or swelling the light-sensitive layer and water must be admixed with one another sufficiently. For this reason, not only it is necessary to use a large amount of surfactants but also the temperature and concentration at which the homogeneous mixed system is stable are limited to very narrow range. In addition, since such a developer simultaneously contains a silicone rubber swelling agent and a developing agent for dissolving or swelling the light-sensitive layer, both properties of these agents become insufficient and the developing ability thereof is also low. Further, a large amount of organic solvent must be used and, therefore, the use of such a developer is not preferable from the viewpoint of environmental pollution and health of operators.
Moreover, conventional light-sensitive layers contain a photodimerizable light-sensitive layer (see, for instance, J.P. KOKOKU No. 55-22781). This makes it difficult to dissolve out non-exposed portions of the light-sensitive layer.
Alternatively, to solve the aforesaid problems, Japanese Patent Application Serial (hereinafter referred to as "J.P.A.") No. 62-92374 proposes a dry PS plate which comprises water-soluble light-sensitive composition and thus can be developed with tap water.
The dry PS plate having a silicone rubber layer as an upper layer wherein a water-soluble light-sensitive composition is used shows extremely high stability since it can be developed with tap water. However, the speed of developing the same is very low since it is developed with a simple tap water.